1. Field of the Invention
The present invention relates to an apparatus for controlling data transfer between storages in an information processing system having a main storage and an extended storage in which the data transfer between the main storage and the extended storage is controlled.
2. Description of the Prior Art
In an information processing system having a main storage and an extended storage, there are two primary usages of the extended storage. First, the extended storage is used by an operating system, for example, for a virtual storage or the extended storage is subjected to a roll-in or a roll-out for each job when a job is to be changed. In such an operation, since it is requested to execute the operation in concurrence with an execution of a user program, the extended storage is generally initiated with an asynchronous instruction (START I/O instruction). Secondly, the extended storage is used by user programs, for example, as a work file. In this case, since the operation must be instantaneously achieved, the extended storage is generally initiated with a synchronous instruction (MOVE instruction).
When these two ways of usage are adopted, there occurs a case where the extended storage is initiated with a synchronous instruction from a user program during an execution of a data transfer caused by an asynchronous instruction. In such a case, according to the conventional technology, the initiation by the synchronous instruction is effected only after the data transfer of the asynchronous instruction is completed. Incidentally, the operations of the extended storage by use of the synchronous and asynchronous instructions have been described, for example, in Japanese Patent Unexamined Publication No. 58-9276. According to the prior art technology, since the extended storage cannot be initiated with a synchronous instruction while a data transfer is being executed by use of an asynchronous instruction, there is a problem that it may occur that an instantaneous data transfer between the main storage and the extended storage, which data transfer is requested by the synchronous instruction, cannot be achieved.